High impact polystyrene polyblends (HIPS) comprising polystyrene having a rubber phase dispersed therein, as crosslinked rubber particles, are known. Historically, mechanical blends were prepared by melt blending polystyrene with raw rubber which was incompatible and dispersed as crosslinked rubber particles to reinforce and toughen the polymeric polyblend. More recently, HIPS polyblends have been prepared by mass polymerizing solutions of diene rubber dissolved in styrene monomer in batch reactors wherein the rubber molecules were grafted with styrene monomer forming polystyrene polymer grafts on the rubber along with polystyrene polymer in situ in the monomer. As the polystyrene-monomer phase increases during polymerization the grafted rubber phase inverts readily as rubber particles comprising grafted rubber and occluded polystyrene contained therein with said particles crosslinked to maintain the rubber particles as discrete particles dispersed in the polystyrene which forms a matrix phase of the HIPS polyblend.
Such HIPS polyblends conventionally contained rubber particles having an average particle size or diameter of about 0.5 to 1.0 microns to provide toughness yet small enough to insure good gloss for molding or sheet applications.
As such HIPS polyblends have moved in engineering applications even greater toughness is needed consistent with good gloss. Efforts to increase the toughness by adding more rubber have met with limited success in that the polymerization processes can only accommodate concentrations up to 15 to 20% and high levels of rubber increase cost and lower gloss. Efforts to increase the particle size of the rubber beyond about 1.0 microns to increase toughness as disclosed in U.S. Pat. No. 4,012,462 has met with limited success because of loss of gloss and flow properties.
It has been discovered that small amounts of larger sized rubber particles can be added to HIPS polyblends without loss of gloss or flow properties providing unexpectedly great increases in toughness beyond the proportionately small amounts of larger particles added to conventional HIPS polyblend having a rubber particle size averaging 0.5 to 1.0 microns.
It is hereby disclosed that the toughness of HIPS polyblends can be increased as much as 50% or more by the novel process of the present invention wherein HIPS polyblends are prepared having a bimodal rubber particle size distribution i.e., 70 to 95% of the rubber particles have an average particle size of about 0.5 to 1.0 microns and 5 to 30% of the rubber particles have an average rubber particle size of 2 to 5 microns.
U.S. Pat. No. 4,146,589 discloses a process for preparing polyblends having a dispersed rubber phase having a bimodal rubber particle size distribution. The cited patent uses first and second reactors operating in a parallel configuration with a first reactor forming a rubber particle size of about 0.1 to 1.0 microns and a second reactor forming rubber particles of about 2 to 3 microns in size. The streams from these two reactors are then mixed in a separate step and finally polymerized in a third reactor to a polyblend having a bimodal rubber particle size distribution.
The method cited in U.S. Pat. No. 4,146,589 has provided an advance in the art, however, it has now been discovered that the toughening efficiency of the bimodal rubber particles can be enhanced greatly if the smaller rubber particles have a longer dwell time in the polymerization system. An improved method has been found possible wherein the smaller rubber particles can be carried through three reaction zones in series while simultaneously forming the larger particles in the second reaction zone to give a polyblend wherein the major rubber phase component of smaller particles has a longer dwell time to form a greater grafted phase and improved the toughening efficiency of the polyblend.